A tissue-stimulating prosthesis consists of at least one electrode for applying electrical stimulation to tissue, and a mechanism for delivering the electrical stimulation to the electrode. Tissue-stimulating prostheses may be internally powered, as in the case of pacemakers, or externally powered, as is the case of a cochlear prosthesis.
In its usual form, as is known in the art, an implantable cochlear prosthesis consists of several parts. An electrode system, such as an array of many individual electrode bands, is inserted into the cochlea and delivers stimulating current to the cochlear tissue. The electrode is connected via a connector to an electronics module (the receiver/stimulator) which generates stimulus currents. In a preferred system, the stimulus delivered to the tissue is a biphasic current pulse with current amplitude up to 2 milliamperes, and duration of each phase up to about 400 microseconds.
An externally worn device (the speech processor) is used to transmit power and information to the implanted receiver/stimulator, and may also receive information telemetered back from the implanted prosthesis. It is the function of the speech processor to generate information as to which electrode should be stimulated, at what amplitude, and when--based on the incoming acoustic signal. This information is transmitted through the skin to the implant at radio frequencies, and the radio frequency energy is also used to provide power to the implant.
The receiver/stimulator and electrode to be implanted are packed and sterilized, usually at the manufacturing plant, and then shipped in a sealed package to the hospital or medical center where the implant surgery is to be performed. The final tests done on the device are carried out at the manufacturing plant before packing and sterilizing. There is a possibility, however, that a failure could occur in the system between the time of final test and the time of surgery, for example, due to shock or jar during handling or shipping. After sterilization, access to the device is impossible without breaking the sterile seal.